1. Field of the Invention
The present invention concerns methods for determining the load condition of a vehicle and methods for monitoring the inflation pressure of tires, which enable the driver to be alerted to the need to adapt the inflation pressure and/or to initiate automatic adaptation of that pressure.
2. Technological Background
For any vehicle fitted with tires, monitoring of the inflation pressure of the tires is essential for ensuring the safety of the vehicle's occupants and that of other road users. For example, if the inflation pressure of a tire is too low, the tire's sidewalls work abnormally and this can result in excessive fatigue of the carcass, elevation of the temperature and abnormal wear: the tire is at risk of irreversible damage, the greatest risk clearly being that the tire may burst during service and that the driver loses control of the vehicle. On the other hand, too high an inflation pressure can result in vehicle stability problems because the contact of the tire with the ground is reduced.
However, it is not possible to specify a single inflation pressure for a given tire fitted on a given vehicle; the pressure depends on the conditions of use and in particular on the load carried by the vehicle: as the load carried by the vehicle increases, the inflation pressure should be increased as well. In effect, the load that can be carried by a tire depends on its internal volume and its inflation pressure. If the pressure/load ratio is correct, the sidewalls of the tire undergo appropriate bending and the service life of the carcass is extended.
The problem arises not just when the tire is being inflated, but also for the continuous monitoring of its inflation pressure. A system aiming to alert a driver to any anomaly of the inflation pressure should in principle take into account the load actually carried by the vehicle: a given inflation pressure can be correct for an unloaded vehicle and insufficient when the vehicle is heavily loaded.
The definition of a predetermined alarm threshold does not overcome that difficulty. If an alarm threshold is set to the minimum inflation pressure that would still be acceptable for a vehicle carrying no load (Pmin (z0)), cases when the inflation pressure is higher than that threshold but insufficient in relation to the actual load carried by the vehicle would not be detected. Conversely, if the alarm threshold is set at the minimum pressure still acceptable for a tire carrying maximum load (Pmin (zmax)), the alarm could be emitted even when the pressure/load ratio is far from being actually unacceptable, in particular when the vehicle is carrying little or no load. Such a threshold would also have the disadvantage of being too close to the pressure recommended for an unloaded vehicle: in effect, tire temperature variations etc. make monitoring impossible with an accuracy better than 0.3 bar. If the difference between the alarm threshold and the inflation pressure recommended for a vehicle carrying no load is smaller than 0.3 bar, reliable and pertinent detection of anomalies is impossible.
What has just been said must also be supplemented by considering the position of the tire on the vehicle. Depending on whether a tire is fitted on the front or rear axle of a vehicle, the load it carries can be very different and can vary greatly as a function of the overall load of the vehicle. In a traction-type vehicle with its engine at the front, the load carried by the tires of the front axle is very significantly higher than the load carried by those on the rear axle when the vehicle is empty, i.e. carrying no load; this load difference decreases as the overall load of the vehicle increases. Conversely, in a propulsion-type vehicle with its engine at the front, the load difference between a front axle tire and a rear axle tire is small when the vehicle is empty and increases as the load carried by the vehicle increases.
This is why vehicle manufacturers generally supply inflation tables that enable users to adapt the inflation pressure to the load carried by the vehicle, taking into account the position of the tires on the vehicle. Those tables generally give at least the inflation pressures recommended for the tires of each axle in two load situations: low load and high load. Thus, correct use of such tables presupposes knowledge of the load currently carried by the vehicle. Ignorance of that load introduces an uncertainty factor in the determination of inflation pressure and consequently tends to reduce the vehicle user's safety margin. It would of course be possible to overcome that difficulty by providing the vehicle with load sensors, but the cost of such equipment is such that most vehicles are not equipped.